feat: Add MIMEType field to ChatMessageImageURL

[kangkangkk]

1. Added MIMEType field to ChatMessageImageURL struct

2. allowing users to specify the MIME type of image URLs in multi-content messages.

3. relevant API doc
   https://platform.openai.com/docs/api-reference/uploads/create

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.61%. Comparing base (5d7a276) to head (609949d).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1095      +/-   ##
==========================================
+ Coverage   99.59%   99.61%   +0.02%     
==========================================
  Files          34       34              
  Lines        2209     1841     -368     
==========================================
- Hits         2200     1834     -366     
+ Misses          6        4       -2     
  Partials        3        3              

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